In recent years, research and development of lithium ion secondary batteries is actively conducted as a high energy-density battery. Lithium ion secondary batteries are expected to be used as an electric source of vehicles such as hybrid vehicles and electric vehicles or as an uninterruptible power supply of mobile phone base stations. However, even if a cell of a lithium ion secondary battery is made larger in size, the voltage obtained from the cell of the lithium ion secondary battery is a low voltage of about 3.7 V. Thus, to obtain a high output from a power supply using the cell of the lithium ion secondary battery, a power supply in which many cells of the lithium ion secondary battery are connected in series needs to be used. As a result, a size of the power supply grows becomes larger.
A bipolar battery is proposed as a cell that can be made smaller in size relative to its output. The bipolar battery uses a plurality of bipolar electrodes, each of which includes a current collector, a positive electrode active material layer formed on one side surface of the current collector, and a negative electrode active material layer formed on the other side surface of the current collector. These bipolar electrodes are arranged with an electrolytic layer being interposed between them and electrically connected in series. Since the plural bipolar electrodes are electrically connected in series in the bipolar battery, high power of a high-voltage and constant current can be obtained and further an electric resistance in the battery is small.
A lithium ion secondary battery uses a liquid electrolyte. And, positive electrodes and negative electrodes are repeated in one cell of the bipolar battery. Thus, if the liquid electrolyte used for the lithium ion secondary battery is applied to the bipolar battery, a short-circuit (liquid junction) may be caused by ionic conduction between the positive electrode and the negative electrode. Therefore, a structure of the cell of the lithium ion secondary battery using the liquid electrolyte cannot be adopted as a structure of the cell of the bipolar battery.
Heretofore, a bipolar battery using a polymeric solid electrolyte that does not include a liquid electrolyte has been proposed. Since the bipolar battery with this structure does not use the liquid electrolyte, there is no possibility of the short-circuit (liquid junction) due to the ionic conduction of the liquid electrolyte between the plurality of bipolar electrolytes. In general, however, an ionic conductance of the solid electrolyte is about 1/10 to 1/100 of that of the liquid electrolyte and is very low in comparison with that of the liquid electrolyte. Therefore, an output density of the bipolar battery in this structure is low and the bipolar battery in this structure is not yet in actual use.
In view of the above circumstances, a bipolar battery using a gel electrolyte obtained by making a liquid electrolyte being semisolid is proposed. The gel electrolyte is produced by soaking an electrolytic solution into a polymer such as polyethylene oxide (PEO), polyvinylidene difluoride (PVdF), etc. Since the gel electrolyte has a high ionic conductivity, an output density of a bipolar battery using the gel electrolyte can also be expected to be high.
There remains a problem in increasing a size of the bipolar battery (that is, in realizing a higher energy density of the bipolar battery). As a method for realizing a higher energy density of the bipolar battery, some methods are considered. In one of the methods, electrode areas of positive and negative electrodes are increased. On the other of the methods, bipolar unit cells each having small electrode areas of the positive and negative electrodes are connected in parallel. In a lithium ion secondary battery having a conventional electrode structure, positive and negative electrodes and separators are spirally wound with no space between them and then are accommodated with high density in a battery case, whereby a higher energy density is achieved. However, in the bipolar battery, owing to its structure, the positive electrode and the negative electrode are integrally formed. Therefore, if the positive electrode and the negative electrode are spirally wound, counter electrodes are in contact with each other. Thus, there is a problem that a short-circuit occurs unless, for example, an insulating layer such as a separator or a polymer is sandwiched between bipolar electrode layers.
However, in this case, a thickness of an electrode body is increased by sandwiching the insulating layer such as a separator or a polymer between the bipolar electrode layers, so that a filling rate of an electrode is reduced. Thus, it is difficult to realize the higher energy density with the use of this method. Further, when the electrode areas is enlarged by the spiral winding, it is difficult to take electrode current collecting tabs which are to be connected to a current collecting terminal portion of the battery, from plural portions. For this reason, as the electrode area increases, an internal resistance of the battery increases and interferes with a realization of a higher output in the bipolar battery. Accordingly, it is required to provide a technique by which the above problems in the bipolar battery are solved and simultaneously a high output/input and a high energy density can be achieved in the bipolar battery.